Hydrocarbon synthesis process using a hydrocarbon synthesis catalyst and an acidic catalyst

ABSTRACT

This invention relates to a hydrocarbon synthesis process comprising the conversion of a feed of H 2  and at least one carbon oxide to hydrocarbons containing at least 30% on a mass basis hydrocarbons with five or more carbon atoms. The conversion is carried out in the presence of an alkali-promoted iron hydrocarbon synthesis catalyst and an acidic catalyst suitable for converting hydrocarbons. The reaction mixture formed during the conversion contains less than 0.02 mol alkali per 100 g iron and the H 2 :carbon oxide molar ration in the feed of H 2  and carbon oxide is at least 2.

TECHNICAL FIELD

This invention relates to a hydrocarbon synthesis process and moreparticularly to such a process wherein a hydrocarbon synthesis catalystand an acidic catalyst is used. The process is particularly, but notexclusively, suitable for producing liquid fuel. The process is alsosuitable for producing hydrocarbons rich in aromatics such as benzene,toluene and xylenes or hydrocarbons rich in branched hydrocarbons orrich in olefins.

BACKGROUND ART

Fischer-Tropsch processes for hydrocarbon synthesis from CO and H₂(syngas) are known to produce gaseous and liquid hydrocarbons as well asoxygenates which, in general, follow the well-knownAnderson-Schulz-Flory product distribution.

These reactions can be carried out in fixed, fluidised or slurry bedreactors. The production of olefins and liquid fuels, especially in thegasoline range products, is most favoured by synthesis carried out in atwo-phase fluidised bed reactor operating at 350° C. and 20 bar orhigher pressures and usually utilising a fused alkali promoted ironcatalyst. This is known as a high temperature Fischer-Tropsch process.

In terms of the ideal Anderson-Schulz-Flory product distribution it isclear that the gasoline (C₅ to C₁₁) and diesel (C₁₂ to C₁₈)selectivities are limited to values of about 48% and 25% respectively,while combined liquid fuels selectivity has a maximum value of around65%. In a high temperature Fischer-Tropsch process performed in afluidised bed reactor, the optimum liquid fuel yield might not berealised, thus resulting in a lower liquid fuel selectivity. In additionto this relatively low liquid fuel yield, the Fischer-Tropsch processhas a further disadvantage in that the product spectrum mainly consistsof linear hydrocarbons. This is a disadvantage with respect to gasolinequality, since linear molecules have a very low octane number.Fischer-Tropsch gasoline thus requires either further work-up to convertthe product to one with a higher octane number, or the addition ofhigh-octane compounds to the gasoline pool.

It is known that the Fischer-Tropsch product spectrum can be worked-upto high octane gasoline range fuels by using an acidic catalyst such asa zeolite catalyst. Such a work-up has the disadvantage that it adds tothe production costs of the liquid fuel.

In order to reduce the above disadvantage it has been attempted tocombine a Fischer-Tropsch catalyst with a zeolite catalyst in order toprepare high octane gasoline range fuels directly from CO and H₂. Insuch a system the idea is that the Fischer-Tropsch catalyst shouldcatalyse the conversion of CO and H₂ to hydrocarbons, and the acidcatalyst should convert the resulting olefinic and oxygenatedhydrocarbons to gasoline range products that are highly branched andhigh in aromatics.

U.S. Pat. No. 4,086,262, U.S. Pat. No. 4,279,830, U.S. Pat. No.4,361,503, U.S. Pat. No. 4,269,783, U.S. Pat. No. 4,172,843, U.S. Pat.No. 4,463,101, U.S. Pat. No. 4,298,695, U.S. Pat. No. 4,304,871, U.S.Pat. No. 4,556,645, U.S. Pat. No. 4,652,538 all disclose the combineduse of a hydrocarbon synthesis catalyst and an acidic catalyst in thepreparation of hydrocarbons from syngas. The two catalytic functionshave been combined in a variety of ways, ranging from a single reactorcontaining both catalytic functions to a dual reactor arrangement withthe two catalytic functions in subsequent reactors. Different reactionconditions and different catalysts are disclosed in these patents.

It is recognised (eg. U.S. Pat. No. 4,298,695) that the addition ofalkali promoters to the iron based Fischer-Tropsch catalyst in abi-functional process (Fischer-Tropsch catalyst and acidic catalyst) isundesirable, since these promoters tend to migrate to the acidiccatalyst with a resultant poisoning of the acid sites.

However, it is known that iron based Fischer-Tropsch catalysts with alow alkaline promoter level tend to produce light hydrocarbons that arenot desirable for gasoline production since they do not fall in thegasoline range of C₅ to C₁₁ and are also not easily converted to thisrange.

Therefore, should a low level of alkali promoter be considered in theproduction of liquid fuel from syngas, the reaction conditions would beselected to produce a heavier hydrocarbon product despite the low alkalilevel. It is well-known that a high H₂:CO ratio in the feed favoursproducts which are not desirable for gasoline production. Accordingly ifa low level of alkali promoter is considered a relatively low H₂:COratio in the feed will be considered to avoid production of excessivelylight hydrocarbons.

Surprisingly it has now been found that if hydrocarbon synthesis ofsyngas is carried out in the presence of a hydrocarbon synthesiscatalyst and an acid catalyst under conditions where:

-   -   1) the hydrocarbon synthesis catalyst includes a low level of        alkali metal; and    -   ii) the hydrogen to carbon monoxide ratio of the syngas feed        stream is relatively high,    -   a hydrocarbon product is produced that is suitable for use as        gasoline. This is true even if the synthesis is carried out        under high temperature Fischer-Tropsch conditions.

DISCLOSURE OF THE INVENTION

According to the present invention there is provided a hydrocarbonsynthesis process comprising the conversion of a feed of H₂ and at leastone carbon oxide to hydrocarbons containing at least 30% on a mass basishydrocarbons with five or more carbon atoms (hereinafter referred to asC₅₊ compounds), the conversion being carried out in the presence of analkali-promoted iron hydrocarbon synthesis catalyst and an acidiccatalyst suitable for converting hydrocarbons; and the process beingcharacterised therein that the reaction mixture formed during theconversion contains less than 0.02 mol alkali metal per 100 g iron andthat the H₂:carbon oxide molar ratio in the feed of H₂ and carbon oxideis at least 2.

The synthesised hydrocarbons preferably contain, on a mass basis, atleast 35%, more preferably at least 40% and most preferably at least50%, C₆ ⁺ compounds. Preferably, the process is for producing liquidfuel, especially gasoline and preferably unleaded gasoline. The processmay also be used for producing hydrocarbons rich in aromatics such asbenzene, toluene and xylenes and/or hydrocarbons rich in branchedhydrocarbons and/or rich in olefins.

The hydrocarbon synthesis process may comprise a Fischer-Tropschprocess, preferably a high temperature Fisher-Tropsch process. Thetemperature range may be between 250° C. and 400° C., typically from300° C. to 370° C., and even from 330° C. to 350° C. The pressure may befrom 10 to 60 bar (1 to 6 MPa), typically from 15 to 30 bar, and usuallyat about 20 bar.

The at least one carbon oxide in the syngas preferably comprises CO. Thecarbon oxide may comprise a mixture of CO and CO₂.

The reaction may be carried out in any suitable reactor. It is foreseenthat it will be carried out in a fluidised bed reactor, preferably in afixed fluidised bed reactor. If the CO hydrogenation (hydrocarbonsynthesis) catalyst and the acidic catalyst are contained on separateparticles, the invention especially suited to a fluidised bed reactor,since there will not be extensive direct contact between the differentcatalysts which will reduce migration of the alkali promoters from thehydrocarbon synthesis catalyst to the acidic catalyst, thereby reducingthe detrimental results associated with such migration.

The hydrocarbon synthesis catalyst may comprise any suitablealkali-promoted iron catalyst suitable for CO hydrogenation butpreferably it comprises a Fischer-Tropsch catalyst. The iron catalystpreferably comprises a precipitated iron catalyst, but it may alsocomprise a fused iron catalyst. It the process is to be performed in afluidised bed reactor, the final catalyst may be produced by means of avariety of known methods in order to obtain particles with acceptablefluidisation properties, such as crushing, spray-drying, etc. In orderto obtain a particle size distribution suitable for fluidisation, thecatalyst may be classified by means of known methods, such as sieving,cyclone classification, etc.

The iron catalyst contains at least one alkali promoter usually if theform of an alkali oxide. The alkali promoter preferably comprisespotassium or sodium oxide. The catalyst may contain more than one alkalipromoter. The alkali can be added to the iron by means various methods,such as impregnation of the iron with the alkali, co-precipitating thealkali with the iron, fusing the iron and the alkali, etc. The totalalkali metal content (mol alkali metal per 100 g iron) must preferablybe below 0.02, more preferably below 0.01, and most preferably below0.005.

The iron catalyst may also contain other promoters. Certain promoters,for example Al, Ti, Cr, Mg, Mn and Ca can be added as structuralpromoters to the iron catalyst. Binders, such as silica or alumina, mayalso be added in case of a spray-dried catalyst.

The acidic catalyst may comprise a zeolite. The zeolite may comprise aHZSM-5 zeolite. If the process is to be performed in a fluidised bed,the final acid catalyst may be prepared in any suitable known way inorder to obtain particles with acceptable fluidisation properties. Forexample, the acid catalyst may be a spray-dried catalyst. A binder (suchas silica or alumina) may also be added to the acidic catalyst.

The silica/alumina ratio of the zeolite can be varied according to theproduct spectrum desired and the lifetime required for the catalyst.

The hydrocarbon synthesis catalyst and the acidic catalyst may becombined in a variety of ways. It is foreseen that in a preferredembodiment of the invention the two catalysts will be contained onseparate particles and preferably contact between the particlescontaining the different catalysts should be limited. This will reducemigration of the alkali promoter to the acidic catalyst. However, it isforeseen that it may also be possible to combine the two catalysts intoparticles containing both catalytic functions. For example, the twocatalysts may be pressed into pellets or spray-dried to produceparticles that contain both catalytic functions. The hydrocarbonsynthesis catalyst may also be supported by the acidic catalyst. Theiron may be loaded onto the acidic catalyst by means of a variety ofart-recognised methods, such as precipitation, impregnation, chemicalvapour deposition, ion exchange, etc.

The ratio (by mass) of hydrocarbon synthesis catalyst to acidic catalystis preferably at least 1.

The H₂ and carbon oxide feed is known as synthesis feed gas (or syngas)and it may also include other components, such as water vapour, Ar, CH₄,light hydrocarbons, etc.

The H₂:carbon oxide molar ratio may be as low as possible at or abovethe ratio of 2. However it may be higher e.g. 2.2 and even as high as2.6; 2.7; 4.4; 4.5, 4.9; 5.0; and 5.8.

The hydrocarbon products of the process may comprise mainly of branchedparaffins and olefins, cyclic paraffins and olefins, and aromatics, butlinear paraffins and olefins may also be present in the productspectrum.

The invention also relates to hydrocarbons produced by the processsubstantially as described hereinabove.

The invention will now be further described by means of the followingnon-limiting examples.

EXAMPLES A

Catalysts

A fused iron catalyst containing low levels of alkali was employed asthe syngas (H₂ and carbon oxide) conversion catalyst. The catalyst wasprepared by adding the relevant promoters to oxidised mill-scale toobtain a dry mixture of the precursor material. This mixture was thenfused in an electric arc furnace at a temperature of about 1650° C. Themolten material was cast as ingots. After cooling, the ingots werecrushed, milled and then sieved to obtain a particle size fraction of 38to 150 micron. The composition of the catalyst is presented in Table 1.TABLE 1 Composition of fused iron catalyst Component Concentration Fe(mass %) 70.8 K (mol/100 g Fe) 0.0064 Na (mol/100 g Fe) 0.0113 SiO₂(g/100 g Fe) 0.78 Al₂O₃ (g/100 g Fe) 0.32

Both K and Na were in the form of their oxides namely K₂O and Na₂O.

The acidic catalysts comprised a “high alumina content” HZSM-5 zeolite(SiO₂/Al₂O₃ molar ratio of 30) and a “low alumina content” HZSM-5zeolite (SiO₂/Al₂O₃ molar ratio of 280) supplied by ZeolystInternational. The Zeolyst International product numbers of the zeolitesare CBV3024E and CBV28014 for the “high alumina content” and “lowalumina content” ZSM-5 zeolites, respectively. The zeolites were in apowdered form and were used as such for the purpose of the microreactorexperiments. The zeolites were supplied in the ammonium form (NH₄ZSM-5)and prior to use they were calcined in air at a temperature of 500° C.for 16 hours to convert them to the acidic form (HZSM-5).

Reactor System

A Berty microreactor was used. The catalyst inside a Berty microreactoris contained in a very thin bed. A fan, situated below this bed,circulates gas through the catalyst at a high rate. The reactor cantherefore essentially be viewed as a short packed bed with an extremelyhigh recycle ratio, and thus behaves approximately like a continuousstirred tank reactor (CSTR). Since the gas circulation through thecatalyst ensures that there are no significant temperature orconcentration profiles across the bed, this reactor is ideal forstudying the highly exothermic high temperature Fischer-Tropsch (HTFT)reaction. It will be appreciated that a Berty microreactor is consideredas a very good simulation of a fluidised bed reactor.

The main component of the Berty reactor feed during the examples was acommercial synthesis gas stream. Pure hydrogen and carbon dioxide wereco-fed from bottles in order to obtain a total syngas feed to the Bertyreactor that was rich in hydrogen. In addition, bottled argon was fed tothe reactor, which served as an internal standard. The flow rates of thefour feed streams were controlled by Brooks mass flow controllers. Thecomposition of the total Berty feed is presented in Table 2. TABLE 2Feed stream composition of Berty reactor experiments (volume %)Component Volume % H₂ 58 CO 12.5 CO₂ 12 CH₄ 5.5 Ar 12

It will be appreciated that the above volume percentages aresubstantially the same as molar percentages since the gasses were at thesame pressure.

The effluent from the Berty reactor was passed through a two-stageknock-out system. In the first pot (hot knock-out pot), waxyhydrocarbons were condensed. The amount of product drained from thispot, if any, was negligible. A second pot (cold knock-out pot) condensedthe condensable hydrocarbons and reaction water, while the uncondensedeffluent flowed to a vent system. The product sampling point wassituated before the cold knock-out pot to ensure that a sample was takenof the comprehensive product spectrum. Samples were taken in glassampoules for later GC analysis. The hydrocarbon product spectrum wascharacterised by means of a GC-FID analysis.

Loading of the Reactor

At the start of the experimental investigation, a baseline hightemperature Fischer-Tropsch run was performed under the same conditions(including catalyst activation) as for the bi-functional catalyst butwith 5 g of iron catalyst (unreduced weight) as the only catalyst.

The process runs according to the invention were performed with 5 g ofiron catalyst (unreduced weight) and 5 g of “high alumina content”HZSM-5 zeolite in the one case and 5 g of iron catalyst (unreducedweight) and 5 g of “low alumina content” HZSM-5 zeolite in the othercase. In both cases an iron catalyst layer was loaded on top of azeolite layer and the layers were separated by a fine wire mesh therebyavoiding contact between the catalysts. It will be appreciated that thecatalysts were present at the same time in the same reactor.

Catalyst Activation and Synthesis

The zeolites did not require any additional activation. The ironcatalyst was reduced with a hydrogen feed of 1000 ml/min for 16 hours ata temperature of 420° C. prior to synthesis. Thereafter, the temperaturewas lowered to 330° C. under a combined feed of argon and hydrogen.After the temperature had stabilised, synthesis commenced by setting allthe mass flow controllers to the desired values in order to obtain thetotal feed composition presented in Table 2 at a total flow rate of 1500ml/min . During reduction and synthesis, the total reactor pressure wasmaintained at 20 bar.

Results

The results of the experiments are presented in Table 3. The productobtained from the process contains a vast number of components. However,only certain compounds that were unambiguously identified were presentedto illustrate the benefits of combining an iron catalyst and an acidcatalyst according to the invention. TABLE 3 Results ExperimentBifunctional Bifunctional Baseline FT run process run process runCatalyst(s) Iron catalyst + “high Iron catalyst + “low Al content” Alcontent” Iron catalyst HZSM-5 HZSM-5 Time on line [h] 22 6  5 (H₂ + CO)conversion [mol %] 46.3 28   42.9 (CO + CO₂) conversion [mol %] 39.729.6 37.8 Hydrocarbon product distribution [mass %] C₁ 11.6 16.8 15.4 C₂10.9  5.5 5.8 C₃ 16.8 11.8 8.2 C₄ 12.8 16.8 15.7 C₅ ⁺ 47.9 49.1 54.9 C₅to C₁₁ fraction 40.7 43.6 53.1 BTXE aromatics selectivity* 0.8 14.8 7.8Aromatic content of C₆-C₈ 3.6 70.9 28 product fraction [mass %] LinearC₅ paraffins/branched C₅ 0.88  0.22 0.45 paraffins [mass/mass] Linear C₆paraffins/branched C₆ 2.24  0.14 0.36 paraffins [mass/mass] Linear C₅olefins/branched C₅ 4.21  0.31 0.32 olefins [mass/mass] Linear C₆olefins/branched C₆ 2.75  0**  0.17 olefins [mass/mass/*This refers to the mass percentage of all the C₆ to C₈ aromatics in thetotal hydrocarbon product spectrum**Due to the small amounts of linear C₆ olefins present in the product,these compounds were not detected by the GC-analysis

From the results it is clear that the process according to the inventionproduced a substantial amount of condensable hydrocarbons, even thoughthe syngas feed to the reactor was rich in hydrogen arid the alkalimetal content of the iron catalyst was low. The C₅ ⁺ selectivity wasabout 50% for both examples of the process according to the invention.

It is further clear that the addition of an acidic catalyst to beFischer-Tropsch process increased the selectivity of the condensablehydrocarbons. The selectivity of the C₅ ⁺ hydrocarbons increasedsignificantly, especially in the case of the “low alumina content”HZSM-5. The increase in the C₅ to C₁₁ fraction, which contains most ofthe gasoline range components, was even more substantial.

The results also indicate that the normal Fischer-Tropsch productconsists mainly of linear hydrocarbons and contains little aromatic andbranched compounds. Since linear hydrocarbons have a very low octanevalue, they are undesirable as gasoline components. By adding an acidcatalyst to the process, the amount of aromatics in the productincreased dramatically and the paraffins and olefins becamesubstantially more branched. These effects are especially noted for thecase of the “high alumina content” HZSM-5.

The examples presented therefore clearly illustrate that the addition ofan acidic catalyst to the Fischer-Tropsch process not only increases theselectivity of compounds that fall inside the gasoline range, but alsothat an improved gasoline fraction, containing substantially higheramounts of high octane value compounds such as aromatics, branchedparaffins and branched olefins, is produced.

EXAMPLES B

The additional examples are intended to demonstrate that the requirementthat the C₅+ hydrocarbon fraction of at least 30 mass % is satisfied atthe extremes of the alkali metal content range (0.005 to 0.01 mol alkalimetal/100 g Fe). It is also intended to demonstrate that thisrequirement is also satisfied at high hydrogen to carbon oxide ratios inthe syngas.

Catalysts

Precipitated iron catalysts containing varying levels of alkali wereemployed as the syngas conversion catalyst. The catalysts were preparedby reverse precipitation. For each preparation approximately 170 ml 25%aqueous ammonium hydroxide (NH₄OH) solution was added to 4.0 ml of an 1M aqueous solution of iron nitrate (Fe(NO₃)₃.9H₂O) while stirring.Precipitation was allowed to occur until a pH of 7. A 0.005 M potassiumcarbonate (K₂CO₃) or sodium carbonate (Na₂CO₃) solution was added to theprecipitate in appropriate amounts to achieve the desired concentrationof promoter in each catalyst. The precipitate was dried at 130° C.overnight, and calcined at 350° C. for four hours. The compositions ofthe catalysts are presented in Table 4. TABLE 4 Composition ofprecipitated iron catalysts Catalyst Fe (mass %) K (mol/100 g Fe) Na(mol/100 g Fe) PS01 68.9 0.0045 0 PS02 68.7 0 0.0045 PS03 69.4 0.009 0PS04 69.0 0 0.009

Both K and Na were in the form of their oxides namely K₂O and Na₂O.

The acidic catalyst comprised a “high alumina content” HZSM-5 zeolite(SiO₂/Al₂O₃ molar ratio of 30) supplied by Zeolyst International. TheZeolyst International product number of the zeolite is CBV3024E. Thezeolite was in a powdered form and was used as such for the purpose ofthe microreactor experiments. The zeolite was supplied in the ammoniumform (NH₄ZSM-5) and prior to use it was calcined in air at a temperatureof 500° C. for 16 hours to convert it to the acidic form (HZSM-5).

Reactor System

The reactor system was as described for examples A.

The feed composition to the reactor was similar to the feed compositionfor examples A (Table 2), but individual gas flow rates were varied toachieve different hydrogen to carbon oxide ratios. The variation in theindividual gas flow rates also caused slight variations in the total gasflow rate. The feed composition and total gas flow rate for each run isgiven in Table 5. TABLE 5 Gas feed flow rate and composition fordifferent experimental runs Total gas H₂ CO CO₂ CH₄ Ar flow rateH₂/(CO + CO₂) (vol. (vol. (vol. (vol (vol Run no. (ml/min) volume ratio%) %) %) %) %) 1 1476 2.7 60.3 13.8 8.3 5.3 12.2 2 1381 4.5 66.3 14.40.3 5.9 13.1 3 1498 2.7 60.4 13.0 9.4 5.2 12.0 4 1335 4.5 66.1 14.2 0.45.8 13.5 5 1590 5.8 71.4 12.0 0.3 4.9 11.3 6 1486 2.6 59.5 13.4 9.7 5.312.1 7 1353 4.4 65.9 14.6 0.4 5.8 13.3 8 1449 4.9 68.2 13.6 0.3 5.5 12.49 1507 2.6 59.6 13.2 10.0 5.3 12.0 10 1365 4.4 66.1 14.5 0.4 5.9 13.2 111474 5.0 68.8 13.3 0.4 5.4 12.2

It will be appreciated that since the H₂, CO and CO₂ were at the samepressure H₂/(CO+CO₂) will be the same whether it is expressed as volumeor moles.

The effluent knock-out, sampling and analysis were as for examples A.

Loading of the Reactor

The reactor was loaded as for examples A.

Catalyst Activation and Synthesis

Activation and synthesis conditions were as for examples A.

Results

The results are presented in Table 6. TABLE 6 The Influence of alkalimetal level and hydrogen/carbon oxide ratio on the selectivity for C₆+(mass %) Alkali metal C₅+ Catalyst Concentration H₂/(CO + CO₂)selectivity No. Alkali (mol/100 g Fe) Run no. ratio (mass %) PS01 K0.0045 1 2.7 41 2 4.5 32 PS02 Na 0.0045 3 2.7 43 4 4.5 35 5 5.8 30 PS03K 0.009 6 2.6 53 7 4.4 46 8 4.9 41 PS04 Na 0.009 9 2.6 56 10 4.4 49 115.0 45

It is clear that at alkali metal levels between 0.0045 and 0.009 molalkali metal/100 g Fe, and at hydrogen to carbon oxide feed ratios of upto 5 or more the requirement of >30% C₅+ selectivity is satisfied.

1. A hydrocarbon synthesis process comprising the conversion of a feed of H₂ and at least one carbon oxide to hydrocarbons containing at least 30% on a mass basis hydrocarbons with five or more carbon atoms (hereinafter referred to as C₅₊ compounds); the conversion being carried out in the presence of an alkali metal promoted iron hydrocarbon synthesis catalyst and an acidic catalyst suitable for converting hydrocarbons; and the process being characterised therein that the reaction mixture formed during the conversion contains less than 0.02 mol alkali metal per 100 g iron and that the H₂:carbon oxide molar ratio in the feed of H₂ and carbon oxide is at least
 2. 2. The process of claim 1 wherein the synthesised hydrocarbons contain at least 40% on a mass basis C₅₊ compounds.
 3. The process of either one of claims 1 or 2 wherein the hydrocarbon synthesis process comprises a high temperature Fischer-Tropsch process.
 4. The process of any one of the preceding claims wherein the at least one carbon oxide in the syngas comprises CO.
 5. The process of claim 3 wherein the alkali-metal promoted iron hydrocarbon. synthesis catalyst comprises a Fisher-Tropsch catalyst.
 6. The process of claim 5 wherein the promotes comprises potassium or sodium oxide.
 7. The process of any one of the preceding claims wherein the acidic catalyst comprises a zeolite.
 8. The process of claim 7 wherein the zeolite comprises a HZSM-5 zeolite.
 9. The process of any one of the preceding claims wherein the hydrocarbon synthesis catalyst and the acidic catalyst are contained on separate particles.
 10. Hydrocarbons produced by the process of any one of claim 1 to
 9. 11. The use of a hydrocarbon synthesis process for the conversion of a feed of H₂ and at least one carbon oxide to hydrocarbons containing at least 30% on a mass basis hydrocarbons with five or more carbon atoms (hereinafter referred to as C₅₊ compound) the process comprising converting a feed of H₂ and at least one carbon oxide to hydrocarbons in the presence of an alkali promoted iron hydrocarbon synthesis catalyst and an acidic catalyst suitable for converting hydrocarbons; and the process being characterised therein that the reaction mixture formed during the conversion contains less than 0.02 mol alkali metal per 100 g iron and that the H₂:carbon oxide molar ratio in the feed of H₂ and carbon oxide is at least
 2. 